chemistry journal of moldova

Gh. Duca et al./Chem.J. Mold.. 2008, 3 (1), 10-21Much data exist nowadays regarding the chemical composition of natural waters, but very little is known aboutthe kinetic laws that describe the interactions between these compounds [10, 39, 40]. A number of systems, such asM 2+ /M + – L – S – O 2/H 2O 2(where, M 2+ /M + variable valence metal in oxidized/reduced form, L – ligand, S – substrate)have been studied. There is a lot of data on thermodynamic properties of metal complexes, reactivity of intermediatefree radicals, mechanisms of activation of O 2and H 2O 2; however very little is known about catalytic oxidation of severalcompounds of significant ecological importance.Among all reducing agents which model the most successfully natural reducing compounds, most extensivelystudied are the ascorbic acid [20], thiolic compounds [27, 28], hydrochinone [27, 29], oxy and hydroxyacids [21, 22].The ascorbic acid is a compound that regulates the intracellular redox state, having a significant medical-biological role.Oxy- and hydroxyl acids form the metabolic cycle of dicarboxylic acids and take part in the exchange of substancesof living organisms with the environment. Hydroquinone is one of the poliphenols very often encountered in surfacewaters, being the precursors of natural humus. Thiolic compounds participate in metabolic processes of microorganisms,are used in the leather industry, in fertilizers technology etc.On the basis of ascorbic acid the redox mechanism of processes within the aquatic environment has beenestablished, with the participation of ions and complexes of copper when the pollutant (P) possesses ligand propertiesor significant reducing properties [20]. During the interaction between the metal and P, a compound with partial chargetransfer is formed (CuDH + ):Cu 2+ (OH - ) + DH - CuDH + + OH -This complex plays the role of a one-electron donator, reducing the peroxide to O H radicals, whereas themetal ion doesn’t change its oxidation state:CuDH + + H 2O 2 Cu 2+ + D + O H + H 2OAnd this lead to the initiation of processes of conjugated oxidation of pollutants in natural water.In the case of small copper ions concentrations, the complex can decompose in products with one-electrontransfer:CuDH + Cu + + DH· ( D + H + )And in the case of high metal ions concentrations, it can interact with the second copper ion, thus oxidizing thedonor by the two-electron pathway:CuDH + + CuOH + 2Cu + + D + H +Given the copper ions concentration in natural waters, which is around 10 -7 M, the one-electron mechanismprevails in these conditions.The discussed system is characterized by a specificity regarding the formation of OH radicals. As opposed to thePhenton system, the generation of O H radicals doesn’t interfere with the valence of the metal, this one remaining inits oxidized form. Copper (I) ions appear in this system as secondary particles, formed as a result of H 2O 2destruction:O H + H 2O 2H O 2H O 2 + Cu 2+ Cu + + O 2+ H +The impact of compounds with don’t exhibit ligand properties in the natural aquatic environment (such ashydroquinone, glyoxalic acid) [28, 44] can be positive or negative, depending on the substrate’s concentration. Duringthe interaction of copper (I) ions with H 2O 2, the particle CuO + results (which can be treated as the hydrolyzed Cu 3+ ion),being the precursor of the O H radical [34]:Cu + + H 2O 2 CuO + + H 2OIn the absence of reactive substrates, this particle oxidizes water, forming OH radicals:CuO + + H 2O (CuOH) + + O HGiven the fact that it is a two-electrons acceptor towards substrates, in the presence of H donors it oxidizes the donoraccording to the two-electrons pathway:CuO + + QH 2 Cu + + Q + H 2OThus, depending on the donors concentration in the environment, processes of inhibition or generation of OH radicalscan take place.Considering all stated above, the presence of reducing agents with no ligand properties has two roles: at smallconcentrations, substrates will nor essentially interfere the process of OH radicals generation, as a result of wateroxidation by CuO + (i.e. CuO + will oxidize intensely the waters molecules and will be less consumed for the QH 2oxidation), at high concentrations, these substances will act as inhibitors, trapping the CuO + particle, which is theprecursor of OH radicals. This is of significant importance for the realization of chemical self-purification processes ofnatural waters, due to the possibility of conjugated oxidation.16